The present invention relates to a scanning optical system for a laser beam printer or the like, and particularly to a multi-beam scanning optical system using a plurality of beams.
Conventionally, a multi-beam scanning optical system has been widely employed. The multi-beam scanning optical system is advantageous in that a plurality of scanning lines can be formed simultaneously. The multi-beam scanning optical system typically includes a plurality of laser sources respectively emitting a plurality of laser beams. The plurality of laser beams are simultaneously deflected by a polygonal mirror. The deflected laser beams passes through an fθ lens, which converges the plurality of laser beams on a surface, such as a photoconductive surface of a photoconductive drum, to be scanned to form a plurality of beam spots. As the polygonal mirror rotates, the beam spots formed on the photoconductive drum move to form a plurality of scanning lines thereon. The direction in which the beam spots move is parallel with the rotational axis of the photoconductive drum. Further, the photoconductive drum is rotated so that the photoconductive surface thereof is two-dimensionally exposed to the plurality of beams.
In this specification, a direction in which the beam spots move (i.e., a direction in which the scanning lines extend) will be referred to as a main scanning direction. Further, a direction in which the surface to be scanned moves with respect to the scanning lines, i.e., the rotation direction of the photoconductive drum will be referred to as an auxiliary scanning direction. In the following description, the shape of optical elements, directions of powers of the optical elements and the like are described with reference to the main and auxiliary scanning directions on the surface to be scanned. That is, if an optical element is described to have a refractive power in the main scanning direction, the power affects the beam in the main scanning direction on the surface to be scanned regardless of the orientation of the element.
In the multi-beam scanning optical system, all the beam spots should move within (i.e., traverse) a width of an imaging area so that the imaging area can be exposed to the beams. If the plurality of beam spots are arranged to align obliquely with respect to the main scanning direction, the scanning lines formed by the plurality of beam spots are shifted with each other in the main scanning direction. In such a case, it becomes necessary to elongate a width of each scanning line so that each beam traverses the imaging area. In order to elongate the scanning lines, it becomes necessary to use a larger polygonal mirror to broaden a deflection angle at which each beam scans. In view of a recent trend of downsizing of the imaging apparatus, it is not preferable to have such a configuration, and the plurality of beams are preferably aligned along a line which is perpendicular to the main scanning direction.
Generally, a scanning optical system is provided with a synchronizing signal detecting optical system for detecting a scanning position of each beam, which is used for controlling an imaging start point of each scanning line.
A typical synchronizing signal detecting optical system includes a photo sensor which detects a laser beam before it enters the imaging area. A predetermined period after the photo sensor detects the laser beam, modulation of the laser beam is started so that the image is formed from the imaging start position (i.e., the upstream end of the imaging area). If all of the plurality of beams are located at the same position in the main scanning direction, all the laser beams are incident on the photo sensor at the same time. Then, a single pulse signal is output by the photo sensor as the synchronizing signal. In such a case, all of the plurality of beams are started to be modulated after the same predetermined period has passed after the output of the pulse signal.
Practically, it is difficult to arrange the plurality of scanning lines at the same positions in the main scanning direction. It is because, all the beam spots are difficult to be aligned at an initial setting stage, and/or due to an external reason such as an oscillation at use, the relative positions of the plurality of beams may be changed to shift from each other in the main scanning direction. If two beam spots are slightly shifted in the main scanning direction, two pulse signals are output by the photo sensor within a very short period of time. In such a case, whichever pulse signal is used as the synchronizing signal, one of the two beam spots is not started to be modulated accurately, and therefore, the imaging start point of one of the two scanning lines is shifted from the predetermined position.